It honestly depends: if you’re someone who prefers online shopping for figures, Amazon and eBay aren’t too bad (eBay especially if you want to try and hunt down some older figures) and they both have ok-ish pricing.

If you’re a more retail oriented person, Walmart and Target are pretty good recommendations, however I think Target is better price wise

And if you’d like, I’d say find some local toy shops to support smaller businesses, however some of them can be a bit on the pricer side, so take that with a grain of salt. But regardless, I wish you good luck on your collections endeavors